Problem: Initially there were 20 cars parked in the grocery store parking lot. During the following 3 hours some more cars parked at the grocery store, and now there are 21 cars in the parking lot. No cars left the parking lot during this time. How many cars parked in the grocery store parking lot after the initial 20 cars?
The number of cars that came and parked at the store is the difference between the number that were parked earlier and the number that are parked now. The difference is $21 - 20$ cars. $21 - 20 = 1$ car.